Addressing a couple with different last names!!

Whether the couple is married or not, 

the woman's name goes first. 

Obviously, if they are married

Mrs. is used before the wife's name.

AND

 "and" proceeds the husband's name on the second line.

EXAMPLE:

Mrs. Jane Doe Brown

and Mr. John Wilson Smith

  

If they are not married, 

you may use Miss or Ms. (your choice), 

but "and" does not proceed the man's name on the second line.

EXAMPLE:

Ms. Jane Doe Brown

Mr. John Wilson Smith